Conventional optical microscopes exhibit a limited space resolution of approximately λ/2, that is, about 300 nm in the case of visible light as a source (λ being the wavelength of the incident light). This limit is given by the Rayleigh criterion, which is defined as the separation between two punctual sources of light, so that the main maximum of diffraction of one will coincide with the first minimum of diffraction of the other.
However, a space resolution higher than this limit may be achieved within the near-field regime. One of the ways to carry out this type of experiment is by using a nanometric probe to collect part of the near-field regime information and transmitting it to the far-field regime. In this way, the probe acts as an optical nanoantenna. In a simplified view, a nanometric optical collector/source scans the sample, generating an image with resolution defined by the size of the source/collector.
The recently worked out Scanning Near-field Optical Microscopy, called also SNOM, is a technique that uses this mechanism to generate images, chemical and structural characterization on the nanometric scale. As a more specific example of this system, the Tip-Enhanced Raman Spectroscopy (TERS), (Chem. Phys. Ltt. 335, 369-374 (2001)) uses the Raman spread in near-field regime to create images of high space resolution.
In spite of its great application potential, the SNOM technique is not yet applied at laboratories due to the difficulty in manufacturing tips with high reproducibility, good optical efficiency, mechanical stability and nanometric apex, which parameters are indispensable to its application as probes in the SNOM system.
Thus, different types of probes have been proposed in the last three decades.
Historically, the first probes were built with optical fibers with tapered end, provided with end opening of tens of nanometers. Other similar probes comprise transparent dielectric tips with metallic covering. The system that uses this type of probe is called aperture-SNOM, pointing out the fact that these probes exhibit an aperture at their apex, through which light is transmitted.
However, these probes have a great disadvantage: the power transmitted by the optical fiber decreases exponentially with their diameter. For this reason, these aperture-SNOM systems exhibit space resolution on the order of from 50 to 100 nm and are still limited by applications where the analyzed signal is very intense, as for example, photoluminescence.
The best resolutions and optical efficiencies achieved were obtained by using apertureless metallic probes, more specifically from noble metals (Hartschuh et al, Phys. Rev. Lett. 90, 2003). On these devices, the physical mechanism that leads to the use of the near-field information differs from the governing mechanism on the probe for aperture-SNOM. Basically, the incident light (source of light or signal of the sample) causes the free electrons of the metallic probe to oscillate collectively and coherently in the interface with the medium. This sharp-pointed electronic oscillation is called surface plasmon. Due to the conical, pyramidal or sharp-tip shape of the probe, the excitation of the surface plasmon in it generates a strong optical effect located at the apex of the probe, due to the high variation in the electronic density at this point. Thus, unlike optical fiber, the smaller the apex of the metallic probe the greater the optical efficiency and the better the space resolution achieved.
The system that uses this type of probe is called scattering-type SNOM (s-SNOM or apertureless SNOM).
Based on this premise, a number of techniques for manufacturing probes for apertureless-SNOM have been developed. The most widely used ones at present are those for electrochemical trimming of gold thread and deposition of silver onto Si tips, manufactured for the AFM technique (Atomic Force Microscopy).
In fact, it is noted that many of the articles published in the near-field optical microscopy area are related to the production and characterization of the scanning devices/probes (Lambelet P. et al, Applied Optics, 37 (31), 7289-72-92 (1998); Ren. B., Picardi G., Pettinger, B., Rev. Sci. Instrum. 75, 837 (2004); Bharadwaj, P., Deutsch B., Novotny, L., Adv., Opt. Photon. 1, 438-483 (2009)).
What aggravates the problem of quality of the probe for SNOM is that its optical efficiency is related to the coupling of the incident light to its surface plasmon. Basically, in the visible and near infrared range, a considerable difference between the wave vector of the incident light and of the surface plasmon inhibits the direct conversion thereof.
However, a way to promote the light/plasmon coupling in an efficient manner would be by using the so-called localized surface plasmon resonance (LSPR). This type of plasmon resonance takes on nanostructures of dimensions smaller than the wavelength of the incident light, thus being submicrometric structure for the visible and near infrared range. Its main characteristics are: (i) its resonance energy that depends strongly on its geometry, and may be scaled proportionally with some dimension of the object; (ii) the resonance may be excited by light directly and efficiently.
Among the various applications for the localized surface plasmon resonance (LSPR), the s-SNOM technique has called special attention due to its high potential for the optical characterization for nanometric scales.
A number of Prior-Art documents describe the use of multiple ways to improve the optical efficiency of metallic massive probes for use in SNOM, as verified in documents Ropers, C., et al., Nano letters. 7, 2784-2788 (2007), entitled: “Grating-Coupling of Surface Plasmons onto Metallic Tips: A Nanoconfined Light Source”. 
However, these documents deal with multiple and equidistant forms, positioned tens of micrometers from the nanometric apex, so that the incident light (laser) is focused on the structures, without illuminating the apex.
CN1031176283, entitled “Micro-Medium Cone and Nanometalgrating-Compound Adopticalprobe”, describes multiple cracks and surfaces made on the outer metallic layer of conical probes made of a dielectric material for use in aperture-SNOM. However, this type of system needs internal dielectric and also an aperture in the apex.
Various patent applications focus specifically on the properties of the SNOM probes, approaching its functioning and, in some cases, its manufacture. This can be verified in the documents cited hereinafter.
U.S. Pat. No. 4,917,462, entitled “Near field scanning optical microscopy”, which describes a probe with aperture in the form of a glass pipette coated with tapered metal that has a tip that enables near-field access of a probe for near-field microscopy. The pipette is formed from a glass tube stretched downwardly into a sharp tip and then coated by evaporation with a metallic layer. The central aperture of the tube is dragged down to a submicrometer diameter, and the metal coating is formed with an aperture at the apex. It also discloses a microscope using the aperture pipette for near-field digitalizing of sample images.
US 2005/0083826, entitled “Optical fiber probe using an electrical potential difference and an optical recorder using the same”. It describes an optical fiber probe that generates an electric potential difference formed between the thin metal layers coated with it, to increase the light transmission rate. The optical fiber probe includes a near-field probe, with a core transmitting incident light from an external light source and having circular cone structure formed at a core end, and is coated on a surface of the circular cone structure to protect the core. The optical fiber probe also includes the metal-coated thin layers on the near-field probe, arranged symmetrically on opposite sides of the near-field probe, and spaced apart from the other to generate the difference in electric potential.
U.S. Pat. No. 4,604,520, entitled “Optical near-field scanning microscopy”. It approaches a near-field optical microscope comprising a “lens” (aperture) fitted with conventional vertical adjustment apparatus and that consists of an optically transparent crystal, having a metal coating with an aperture at its tip with a wavelength diameter of the light used to illuminate the object. Connected to the farthermost end of the “lens” aperture is a photodetector through an optical filter and an optical glass fiber cable. The digitalization of the object is made by moving adequately the support along the coordinates x and y. the resolution obtained with this microscope is about 10 times as high as that obtained on prior-art microscopes.
US 2010/0032719, entitled “Probes for scanning probe microscopy”. It embraces probes for scanning microscopy comprising a semiconductive hetero structure and the methods of making the probes. The semiconductive hetero structure determines the optical properties of the probe and enables optical image with nanometric resolution.
WO 2009/085184, entitled “Protected metallic tip or metallized scanning probe microscopy tip for optical applications”. It relates generically to a probe for microscopic scanning with a protected metallic tip for applications in near-field optical scanning microscopy with closed tip that comprises a metallic tip or a metallic structure that covers a tip of the microscopic scanning probe, protected by an ultrafine dielectric layer. The protection layer is constituted by SiOx, Al2O3, or any other hard and ultrafine dielectric layer that prolongs the useful light of the tip, providing thermal, mechanical and chemical protection for the whole structure.
However, these probes from the prior art do not have reproducibility of optical efficiency. Some authors report that, even by selecting morphologically adequate gold tips, only 20% proved to be optically active, that is, could be used as probes in SNOM (Hartschu et al, J. of Microscopy, 210, 234-240 (2003)).
Thus, the present invention proposes a solution to the problems of the prior-art probes by providing a probe such as a metallic device that exhibits at least one trimming close to its apex.
The matter dealt with herein comprises a metallic device for use preferably as a near-field optical microscopy and spectroscopy. The proposed device comprises, in general, a single body that has, at its surface, at least one trimming with adequate dimensions and details, which enable the best coupling with the electric field of the polarized light, preferably in the direction normal to the surface to be analyzed. With adjustment of the trimming position with respect to the probe apex, the device enables one to tune the absorption to the frequency of the light used in the system of the desired application, by obtaining proper conditions to generate localized surface plasmon resonance (LSPR) with specific energy, leading to the improvement of the optical efficiency. The object proposed herein has reproducibility with regard to the efficacy in optical absorption and scattering at its apex, and can analyze structures of nanometric dimensions with high resolution.